I find that people tend to let down all their guards when things are festive, for example, at a Fourth of July barbecue and other similar gatherings. People leave cars wide open, leave valuables laying around, leave doors unlocked and wide open, etc.

Now that your "cherry has been popped", it will be shame on you if this same type of thing happens to you again. You don't have anymore passes to lounge around happy-go-lucky without any situational awareness. I recommend to be more aware generally. Lock doors. Keep an eye out. Get a good watch dog. Fire your current watch dog.

Then again, at some point, you can't totally avoid a highly determined thief. If the thief is determined enough, they'll eventually get you. As a practical matter, you can't eliminate the possibility of all theft. Hopefully, you'll only have to worry about petty thieves.